Remove sgid from .nix-portable

This should prevent issues when putting it under /gpfs/{projects,scratch} that have sgid.

pkgs/nix-portable/default.nix

@@ -151,6 +151,11 @@ let
     [ -z "\$NP_LOCATION" ] && NP_LOCATION="\$HOME"
     NP_LOCATION="\$(readlink -f "\$NP_LOCATION")"
     dir="\$NP_LOCATION/.nix-portable"
+
+    # Create NP_LOCATION and remove sgid bit
+    mkdir -p \$dir
+    chmod g-s \$dir
+
     store="\$dir/nix/store"
     # create /nix/var/nix to prevent nix from falling back to chroot store.
     mkdir -p \$dir/{bin,nix/var/nix,nix/store}